FXUS63 KLSX 180903 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 403 AM CDT Mon May 18 2026 .KEY MESSAGES... - Widespread showers and thunderstorms are expected throughout the day with heavy rainfall in some locations potentially leading to flash flooding. - A few strong to severe thunderstorms are possible this afternoon and evening with the main hazards being hail and damaging wind gusts. - Additional thunderstorms are forecast late tonight and early Tuesday morning with another round possible along a cold front Tuesday afternoon/evening. && .SHORT TERM... (Through Late Tuesday Afternoon) Issued at 402 AM CDT Mon May 18 2026 A complex of thunderstorms, that as of 09z, is currently positioned near Quincy MO to Warrensburg MO. This activity initiated across KS/NE on Sunday evening and has continued to push southeastward early this morning, with arrival in central/northeast MO expect around 09-10z. Within the last few hours, this complex has become outflow dominant, which has led to a gradual weakening trend from the severe wind gusts it was producing across northwestern MO around midnight. Upstream observations indicate that the gust front has been achieving winds gusts in the 30-50mph range. As a result, the main severe hazard tied to these storms as they move into the area are expected to be scattered wind gusts up to 50mph, along with frequent lightning and heavy rain. In fact, one of the main concerns regarding the short term is now the threat for heavy rainfall and flash flooding, mainly across central/south-central MO and points west. CAMs are in good consensus that the southeastward progress of this thunderstorm complex will continue to slow as it makes its way into the area this morning. Additionally, there are signs that backbuilding of thunderstorms may occur as the remnant outflow boundary/line of convection stall in a west-east orientation, with strong low-level southerly warm/moist advection overrunning this stationary boundary. Analysis of forecast soundings unveils a plentiful amount of uniform CAPE (around 1,500 J/kg) throughout the column, along with warm cloud temperatures with freezing levels around 12-14kft AGL. In addition, PWATs are currently in the 1.4-1.5" range, with the latest HREF indicating values increasing to 1.7-1.9" today, which is near the 99th percentile. Lastly, the HREF LPMM precipitation reveals the potential for a west-east oriented swath of 2-4" with localized pockets of 4-6" possible. Given all of the variables mentioned above, the risk of heavy rainfall has prompted us to issue a Flash Flood Watch for central/south-central MO and portions of east- central MO that is in effect from 12z today through this evening. Regarding the severe weather potential throughout the day, the threat remains for an isolated thunderstorm becoming strong/severe with hazards being hail along with damaging winds. This potential appears to be lower due to the lingering showers and thunderstorms across a large portion of the area, inhibiting solar insolation, and thus, robust destabilization this afternoon. Regardless, given the warm/moist summerlike airmass in place and the continuation of steepened mid-level lapse rates, maintaining sufficient instability, the threat for an isolated severe storm remains. A few locations may be able to clear by this afternoon, allowing the environment to recover and destabilize, and potentially the threat for strong to severe thunderstorms to increase in localized areas. With uncertainty regarding the evolution/coverage of lingering showers/storms, confidence is low regarding the extent of severe weather that may be realized across the area this afternoon and evening. By tonight, the large-scale pattern features a broad mid/upperlevel trough to the west, with an embedded shortwave ejecting northeastward into the Great Plains. This mid-level feature triggers lee-side surface cyclogenesis across the Central Plains/Midwest near a quasi-stationary baroclinic zone. Convection is modeled by high resolution guidance to initiate along this boundary in KS/NE, with quick upscale growth ensuing with eastward progression. The surface low is modeled to lift northeast into the Midwest on Tuesday, which begins to slowly shift the quasi-stationary baroclinic zone southeastward as a cold front. However, this convection is progged by guidance to outrun this slowly advancing cold front out west, and gradually weaken as it approaches the area. This weakening trend appears reasonable since the best deep-layer shear remains displaced to the north and west of the area and forcing for ascent will be limited without convection tied to the boundary. Regardless, this threat for severe weather late Monday night into early Tuesday morning will include the potential for large hail and damaging wind gusts. Additionally, if surges or bowing segments occur, then severe outflow gusts or embedded circulations leading to brief tornadoes may also be possible. Guidance is in fairly good agreement that the cold front will approach northeast MO/west-central IL Tuesday morning with it exiting to the southeast late Tuesday evening. Locations that remain pre-frontal for a majority of the day on Tuesday will have the greatest severe weather threat. The big question that remains is, how quickly will the atmosphere recover from the early Tuesday morning convection by the time the front is working its way through the area. As of now, it appears the severe weather threat will be greatest from east-central MO and points southeastward, as these locations will have the best chance to destabilize in the early afternoon on before the arrival of the front. Regardless, instability and mid-level flow are still forecast to be sufficient enough to support organized storms with potential for large hail, damaging winds, and tornadoes. Any lingering outflow boundaries from morning convection may serve as areas of focus for thunderstorm development in the afternoon ahead of the cold front. Peine && .LONG TERM... (Tuesday Night through Sunday) Issued at 402 AM CDT Mon May 18 2026 Low-level cold air advection within the post-frontal airmass will lead to much cooler conditions for Wednesday, with highs forecast to be around 10-15 degrees below average. Quasi-zonal flow aloft encompasses the region through the end of the week, which should allow for a period of dry conditions until Friday. By the end of the week, the general trend of LREF temperature IQRs is a gradual warmup to temperatures right around climatological normals. Peine && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1022 PM CDT Sun May 17 2026 Showers and thunderstorms line up from Minnesota through Kansas late this evening. This activity is expected to be the primary source for thunderstorm potential heading into Monday with the main impacts being MVFR visibilities, locally heavy rain, and gusty winds. However, the intensity of this line is in question as it moves southeast through Missouri and into Illinois early Monday morning. Another disturbance drive northeast out of the southern Plains, which could lead to a few showers and isolated thunderstorm ahead of the approaching line. The isolated nature of what develops ahead of the line makes it difficult to pinpoint direct impact to any one terminal and therefore is not mentioned in the TAF at this time. The strategy was to focus on TSRA along the main line and target best potential for TSRA impacts. KUIN has the greatest chances (70-80%). KCOU/KJEF lie at the southern end of theline, where thunderstorms are likely to weaken, but maintain a broad spread of showers with embedded thunder potential. It remains uncertain whether thunderstorms will maintain enough intensity through the metro terminals, justifying the continuation of PROB30 groups to account for TSRA. One trend that will need to be monitored is where outflow stalls Monday. A second, weaker surface low ejects from the Plains along the boundary, which could support showers and isolated thunderstorms over central MO/metro sites through much of the day. It would be reasonable to extend the -SHRA mention with PROB30s for TSRA, if trends continue in this direction. Otherwise, conditions largely vary between MVFR/VFR through the period with MVFR concentrated on relatively more intense SHRA/TSRA. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch through this evening for Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-Moniteau MO- Montgomery MO-Osage MO-Warren MO-Washington MO. IL...None. && $$ WFO LSX